1. Field of the Invention
The present invention relates to a mobile communication system, communication control method and a mobile station, and particularly relates to improvement of the mobile communication system which uses the mobile station as dual mode terminal equipment and can realize the dual mode communications in different radio communication systems.
2. Description of the Related Art
In recent years, in a mobile communication system represented by the cellular phone system, users are increasing sharply by its convenience, and also with respect to a communication method used for the mobile communication system, a wide variety of radio communication systems are proposed and put in practical use.
In these circumstances, there is a mobile station of what is called dual mode terminal equipment, which has a capability of communicating in a plurality of communication systems, and technologies of such a dual mode mobile station are disclosed in several Japanese patent documents.
The mobile station of dual mode terminal equipment indicated by JP, 2004-032337, A is that the mobile station usually operates with the radio base station of a time division multiple access (TDMA) system and during a standby state, it receives the positioning information from the GPS (Global Positioning System) and searches a database based on the position information, and if it finds the radio base station of a code division multiple access (CDMA) system exists near the mobile station and radio transmission condition is better than the TDMA system, the mobile station changes the radio base station from the TDMA system to the CDMA system for communication.
The mobile station of dual mode terminal equipment indicated by JP, 2004-297459, A is that the mobile station changes over mobile communication systems in accordance with the surrounding radio environment where the mobile station is placed, and the mobile station has a function of measuring received electric field level of radio signals from other mobile communication system when the received electric field level of radio signals from the current mobile communication system having degraded by a fixed threshold level. By this function, a changeover of communication systems can be smoothly performed from one mobile communication system to other communication system.
The mobile station of dual mode terminal equipment indicated by JP, 2004-297490, A is the mobile station with dual mode operation for mobile IP with respective radio equipments for a wireless LAN and a cellular phone network, and performs the data communications in mobile environments. In this technology, an IP address assigned by each network is managed to identify the same mobile station. Therefore, a home agent manages the IP address in each network with the combination of the mobile station identification and the radio equipment identification, and allows the mobile station to communicate in the different networks flexibly.
The conventional mobile station with dual mode capability needs registration of its location in a plurality of mobile communication systems because it should perform standby operation of waiting for a call from respective networks, and for this purpose, it performs intermittent reception of paging messages from respective networks as the standby operation. Therefore, the mobile station with dual mode capability requires bigger power consumption during the standby state compared with conventional mobile station (with a single mode capability), and it causes a problem that a battery of the mobile station with dual mode capability does not last longer than conventional mobile station.